1. Field of the Invention
The present invention relates to electrolytic plating solutions which have particular utility for uniformly depositing a metal coating on the walls of printed circuit board through holes and on the surfaces of such boards.
2. Background Art
Methods for electroplating articles with metal coatings generally involve passing a current between two electrodes in a plating solution where one of the electrodes is the article to be plated. A typical acid copper plating solution comprises dissolved copper (usually copper sulfate), an acid electrolyte such as sulfuric acid in an amount sufficient to impart conductivity to the bath, and proprietary additives to improve the uniformity of the plating and the quality of the metal deposit. Such additives include brighteners, levelers, surfactants, suppressants, etc.
Electrolytic copper plating solutions are used for many industrial applications. For example, they are used in the automotive industry to deposit base layers for subsequently applied decorative and corrosion protective coatings. They are also used in the electronics industry, particularly for the fabrication of printed circuit boards. For circuit fabrication, copper is electroplated over selected portions of the surface of a printed circuit board and onto the walls of through holes passing between the surfaces of the circuit board base material. The walls of a through hole are first metallized to provide conductivity between the board's circuit layers.
Early efforts to make circuit boards used electrolytic copper plating solutions developed for decorative plating. However, as printed circuit boards became more complex and as industry standards became more rigorous, solutions used for decorative plating were often found to be inadequate for circuit board fabrication.
To provide a high quality and uniform metal deposit, it has been recognized that the concentration of several of the ingredients of the electrolytic plating solution (including brighteners and leveling agents) should be kept within relatively close tolerances during the plating process. It should be appreciated that the use of brighteners and levelers in an electroplating bath can be crucial in achieving a uniform metal deposit on a substrate surface.
Prior methods for controlling the concentration of electroplating bath components such as brighteners and levelers have included regular additions of the particular components based upon empirical rules established by experience. Such an approach has some notable and obvious shortcomings, however, as depletion of the bath components is not always constant with time and bath use. Another prior art method is to plate articles or samples and visually evaluate the plating quality to determine if the bath is performing satisfactorily. More specifically, in standard Hull Cell and "Bone Pattern" tests, a specially shaped test specimen is plated and then evaluated to determine the quality of the deposit. This is a relatively time consuming test which typically gives only a rough approximation of the concentration of the bath constituents. Other methods for evaluating the quality of an electroplating bath have been reported in U.S. Pat. No. 4,132,605, and Tench and White, J. Electrochem. Soc., "Electrochemical Science and Technology", 831-834 (April 1985), both incorporated herein by reference.
In pending and commonly assigned U.S. patent application, Ser. No. 07/666,798, filed Mar. 8, 1991 (incorporated herein by reference and sometimes referred to herein as "said pending application"), a novel method is disclosed for determining the quantity of brighteners and levelers present in an electroplating bath. The method of said pending application monitors changes in energy output of the system over time for specific steps in the plating process. The method is based on differences in adsorption behavior of brighteners and levelers on metals. This differential adsorption behavior allows for controlled adsorption of first the brightener and then the leveler in two distinct steps. During the equilibration step, when no current flows, the organic brightener compounds are much more readily adsorbed on a metal electrode compared to the leveler compounds. The adsorption step is carried out for a time necessary to determine the concentration of the brightener. An optional electroplating pulse step can be used before or after equilibration to increase sensitivity or to shorten equilibration time. After the equilibration step, metal is plated, first to measure brightener concentration, and then the rate of change of energy output from the system is recorded in order to determine leveler concentration. The initial potential recorded during this step is a measure of the brightener concentration. When the energy output is plotted versus time, the slope of the line indicates the ratio of brightener to leveler present in the bath. The sensitivity of this process allows for determination of organic additive concentrations down to 1 ppb. As used herein, the term "ppb" refers to parts per billion, and the term "ppm" refers to parts per million.
Plating a substrate having irregular topography can pose particular difficulties. During electroplating a voltage drop variation typically will exist along an irregular surface which can result in an uneven metal deposit. Plating irregularities are exacerbated where the voltage drop variation is relatively extreme, i.e., where the surface irregularity is substantial. Consequently, high quality metal plating (e.g., a bright metal plate of substantially uniform thickness) is frequently a challenging step in the manufacture of printed circuit boards. Printed circuit boards often have "through holes", perforations through the board surface to provide attachment means for the board hardware and, in the case of both double-sided and multilayer boards, to provide interconnection between the board's circuit layers. For multilayer or double-sided boards, through hole walls are first metallized with copper before electroplating to provide conductivity between the two surfaces of the board and multiple circuit innerlayers of the board when they are present. Processes for the formation of conductive through holes are well known and described in numerous publications such as U.S. Pat. No. 4,515,829.
As may be evident from the foregoing, electrodepositing a uniform metal plate becomes more difficult in direct proportion to circuit board through hole geometry, i.e., the circuit board difficulty. Circuit board difficulty is defined to mean herein the thickness of the board multiplied by the ratio of the length of the board's through holes to the hole's diameter (known as the aspect ratio). As board difficulty increases, the voltage drop also increases between the plane surface of the board and the midpoint of a through hole. This voltage drop can result in plating irregularities including "dog boning", i.e., metal plates of uneven thickness on the through hole walls with the metal deposit thicker at the top and bottom of the holes and thinner at the center. The thin deposit at the hole midpoint can result in circuit defects and board rejection. Notwithstanding such problems associated with plating high difficulty circuit boards, the circuit board industry continuously seeks greater circuit densification and, hence, multilayer printed circuit boards of increased thickness (i.e., increased circuit layers) and difficulty.
Consequently, electroplating solutions that can provide good "throwing power" over irregular topography are highly desirable. In the case of a printed circuit board, throwing power of a plating solution has been defined as the ratio of current flowing at the center of a through hole of the circuit board to the current flowing at the board surface during use of the plating solution. See U.S. Pat. No. 5,051,154, incorporated herein by reference. Another measure of the throwing power of a plating solution is the ratio of the thickness of metal deposited in the mid-barrel of a through hole by the solution to the thickness of the metal plated at the circuit board plane surface, e.g., on the through hole's surface pad. An increase in a plating solution's throwing power can obviate "dog boning" and other plating irregularities along a through hole wall.
It thus would be desirable to have a copper electroplating solution that was useful for plating substrates having irregular topography. It would be particularly desirable to have a copper electroplating solution that could plate uniform copper deposits on through hole walls of high difficulty circuit boards.